1. Field of the Invention
This invention relates to a method for measuring the concentration of gases and vapors in a gaseous mixture. More particularly, this invention relates to a method for measuring the concentration of oxygen in a gaseous mixture. This invention is particularly useful for measuring oxygen concentration in hot, dirty exhaust gases from combustion processes. In addition, this invention provides a means for real-time control of combustion processes based upon the concentration of oxygen or combustion-derived gases or other gases and vapors in the exhaust gases of the combustion processes.
2. Description of Prior Art
There are three basic methods for measuring the concentration of oxygen in a gas mixture in commercial use. However, due to their cost, the inherent time delay in obtaining the results and their unreliability in harsh industrial environments, which typically include a combination of heat, corrosive gases, dust and the like, none of these methods is rugged, low-cost or reliable enough to provide input to combustion control systems.
Paramagnetic oxygen analyzers are commonly used in continuous emissions monitoring systems. These analyzers measure the paramagnetic susceptibility of the sample gas by means of a magnetic-dynamic type measuring cell. While this technique is accurate and reliable, disadvantages of these instruments include the need for regular calibration and a response time that is too slow for control applications. In addition, the cell must be maintained at a constant temperature, for example 50xc2x0 C., and the gas must be dry. In practice, exhaust gas samples must be cooled and dried before being sent to the analyzer, which results in significant time delays.
Commercially available electrochemical analyzers capable of directly measuring oxygen concentrations in hot exhaust gases utilize zirconium dioxide (ZrO2) as a solid electrolyte and platinum, NiCr and/or other compounds, as electrode materials. The anode is exposed to a reference gas while the cathode is exposed to the sampled gas stream. Zirconia is an ionic conductor at temperatures above 600xc2x0 C. As a result, variation in the electrochemical potential of the cell reflects variation of the oxygen content of the sampled gas. While response time is rapid, these analyzers have several disadvantages including high labor maintenance costs, the use of expensive materials required to make the sensors, and a short service life of only six to 12 months in harsh environments.
A serious disadvantage of traditional continuous emissions monitors is a substantial time delay between the moment of combustion and when the results of the analysis are complete because the sample must be extracted from the flue gas stream, dried and finally analyzed. This makes the implementation of continuous emissions monitors for burner control very difficult. Optical sensors are capable of overcoming this problem. Narrow-band optical detection of intermediate species within the burner flame overcomes this time lag. In flame analyzers, different wavelength radiation sensors, filters and data acquisition and processing systems are combined to measure concentrations of a number of radicals formed during the combustion process including OH, CO, and CH. This information is used to determine the air/fuel ratio and the presence of soot. See, for example, U.S. Pat. No. 5,741,711, which teaches a method and apparatus for analyzing a sample by introducing the sample into a combustible mixture, igniting the combustible mixture to produce a flame, and detecting a characteristic of the resulting flame to determine the identity and/or concentration of one or more chemical substances in the sample, wherein the combustible gas mixture is generated by water electrolysis. The apparatus includes an inlet for introducing combustible gases, a feeder for introducing the sample into the combustible gases, an ignitor for igniting the combustible gases to produce a flame, a detector for detecting a characteristic of the resulting flame for determining the identity and/or concentration of one or more chemical substances in the sample, and a water electrolyser for generating combustible gases and for directing the gases to the inlet. This technique has a number of serious problems and limitations in industrial practice. System calibration is difficult, and there is typically strong interference from refractory or wall radiation. Flame turbulence requires sophisticated data processing to separate signals from noise. In addition, the radiation spectrum coming from real, industrial scale, flames often is estimated as black body radiation spectrum making separate radiation intensity measurements associated with detectable radicals difficult.
U.S. Pat. No. 5,708,507 teaches a method and apparatus for temperature resolved molecular emission spectroscopy of solid, liquid or gaseous materials in which a sample is vaporized and decomposed, and the vaporous sample is then transported into a combustion flame. A spectrum of intensity in the optical emission from the flame at a selected wavelength versus temperature of the sample defines molecular peaks which are characteristic of the sample material and allows both qualitative and quantitative analysis of the sample. See also U.S. Pat. No. 3,917,405 which teaches the use of a flame photometric detector for analysis of a sample burned in a flame and U.S. Pat. No. 3,609,042 which teaches an optical measuring apparatus for sampling material in which the samples are introduced into a flame and light beams which pass through the flame are detected by a detector which, in turn, produces electric signals corresponding to the concentrations of the samples within the flame.
Another method and apparatus for determining the concentration of an analyte such as oxygen in an unknown gas sample is taught by U.S. Pat. No. 6,091,504 in which a vertical cavity surface emitting laser is used as a variable wavelength light source which is xe2x80x9csweptxe2x80x9d through a wavelength range by varying the drive signal applied thereto. The variable wavelength light source is repeatedly xe2x80x9csweptxe2x80x9d through a range of frequencies determined by the drive signal, and the absorption is measured by the detector.
Notwithstanding the number of known methods and devices for analyzing the content of a gaseous mixture, none of them provide real-time analysis whereby the results may be employed to control an application, such as a combustion process. In addition, known methods and devices do not simultaneously measure oxygen concentration and the concentrations of other gases, including CO, total hydrocarbons, NOx and the like in static and flowing gases, including harsh industrial exhaust gas streams.
Accordingly, it is one object of this invention to provide a method and apparatus for measuring the oxygen concentration of gaseous mixtures in harsh industrial environments.
It is another object of this invention to provide a method and apparatus for measuring the concentrations of oxygen and other gaseous and vaporous components of a gaseous mixture which is not affected by the presence of contaminants including dust, halides, NOx and SOx.
It is yet another object of this invention to provide a method and apparatus for simultaneous measurement of O2, CO, total hydrocarbons and other gaseous and vaporous components of flue gases.
It is yet a further object of this invention to provide a method and apparatus for measuring the oxygen concentration of a gaseous mixture which does not require frequent calibration.
These and other objects of this invention are addressed by a method for measuring the concentration of at least one gaseous and/or vaporous component of a gaseous mixture comprising the steps of introducing a controlled sensor flame into the gaseous mixture, optically measuring at least one narrow spectral band in the controlled sensor flame, and calculating a concentration of the gaseous and/or vaporous component using the result obtained from the optical measuring of the at least one narrow spectral band, the narrow spectral band being less than 20 nanometers, and preferably less than 10 nanometers, in width. Although similar to known methods for measuring the gaseous content of a gaseous sample, the method of this invention is distinguishable in that it does not involve or require the introduction of the sample directly into the flame. For measuring the oxygen concentration of flue gases, the observed oxygen concentration data is acquired sufficiently close to the flame, soon enough after the combustion process, and reliably enough to be used by a combustion control system to optimize combustion efficiency and minimize emissions in substantially real time. Conventional oxygen sensors and methods for determining oxygen concentration in a gaseous stream do not have these capabilities.
Advantages of using controlled flames as a means for monitoring combustion emissions include the ability to operate reliably and continuously for extended periods of time in harsh industrial environments and simultaneous measurement of oxygen, carbon monoxide, total hydrocarbons and even NOx. In addition, the presence of contaminants including dust, halides, NOx and SOx does not impair operation and the equipment required to carry out the method is simple, inexpensive and does not require continuous calibration. Yet a further advantage of the method and apparatus of this invention is the range of gas mixtures that can be analyzed for oxygen concentration. Measuring the oxygen concentration of xe2x80x9cdirtyxe2x80x9d, hot gases such as exhaust gases from industrial combustion processes using conventional means is difficult. This invention provides a means for determining oxygen concentrations of these difficult gas mixtures and for measuring oxygen concentration in other static and flowing gas mixtures at less stringent conditions.
Finally, this invention offers significant advantages to the operators of industrial combustion processes because the oxygen concentration data from the dirty, hot exhaust gases immediately downstream of a furnace provides direct information regarding the combustion process. The data available from conventional systems only provide indirect information about the combustion process. The immediate availability of the oxygen concentration in the dirty, hot exhaust gases enables the combustion process control system and operators to optimize the combustion process in real time.